Due to environmental problems and resource problems, also in tires for vehicles, low heat buildup property is strongly required in recent years. A tire obtained by using a rubber composition in which silica is mixed is more excellent in the low heat buildup property than a tire obtained by using a rubber composition in which usually used carbon black is mixed, and therefore, a fuel efficient tire can be produced by using this composition. On the other hand, even if usually used rubber is mixed with silica, the rubber is poor in the affinity with silica and hence is easily separated, and as a result, it is apprehended that the fuel efficiency may be degraded.
For such a rubber composition, there is a known art, to be employed for improving the affinity between the rubber and silica, in which a functional group having high affinity with silica is introduced by reacting a polymerization active end or the like of the rubber with a modifier.
For example, Patent Document 1 discloses a method of production of a radial conjugated diene polymer which comprises polymerizing a monomer mixture containing at least a conjugated diene compound by using, as a polymerization initiator, an alkali metal-reacted aromatic compound having three or more carbon atoms directly bonded to an alkali metal atom and an aromatic ring in one molecule. In Patent Document 1, since the conjugated diene polymer has a radical structure, if a filler such as silica is added thereto, the affinity with the filler can be improved so that low heat buildup property and abrasion resistance can be improved. Furthermore, in the art described in Patent Document 1, the radical conjugated diene polymer is further improved in the low heat buildup property by using a modifier having an alkoxy group, but gelation (three-dimensional cross-linking) is caused in the art of Patent Document 1 by the modification reaction unless a modification point is adjusted, which causes a problem of inferior processability, and therefore, there are demands for further improvement.